CHAPTER XXV. 

 IMMUNITY. 



Immunity, as has already been stated, implies such a con- 

 dition of the body that pathogenic organisms after they 

 have been introduced are incapable of manifesting them- 

 selves and are unable to cause disease. The word has come 

 to have a more specific meaning than resistance in many 

 instances, in other cases the terms are used synonymously. 

 It is the opposite of susceptibility. The term must be 

 understood always in a relative sense, since no animal is 

 immune to all pathogenic organisms, and conceivably not 

 entirely so to anyone, because there is no question that a 

 sufficient number of bacteria of any kind might be injected 

 into the circulation to kill an animal even though it did it 

 purely mechanically. 



Immunity may be considered with reference to a single 

 individual or to entire divisions of the organic world, with 

 all grades between. Thus plants are immune to the diseases 

 affecting animals; invertebrates to vertebrate diseases; cold- 

 blooded animals to those of warm blood; man is immune 

 to most of the diseases affecting other mammals; the rat to 

 anthrax, which affects other rodents and most mammals; 

 the well-known race of Algerian sheep is likewise immune 

 to anthrax while other sheep are susceptible; the negro 

 appears more resistant to yellow fever than the white; some 

 few individuals in a herd of hogs always escape an epizootic 

 of hog cholera, etc. 



Immunity within a given species is modified by any of the 

 following: age, state of nutrition, vitamine deficiencies, ex- 

 tremes of heat or cold, fatigue, excesses of any kind. In fact, 

 anything which tends to lower the ''normal healthy tone" of 

 an animal also tends to lower its resistance. Children appear 

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